PDF is a common format used to distribute content online, and
provides more of a "digital paper" feel than more flexible formats
like HTML. This determinism is why tools like LaTeX work well
with PDFs. Unfortunately, the PDF specs have evolved over the years,
incorperating more of HTMLs "features" like JavaScript (PDF 1.3,
2000), and as the specs get more complicated, PDF readers get larger
and more complicated, and there are more possiblities for security
holes.